Conventional metal machining methods include electrolytic forming methods. In such electrolytic forming methods, the gap between an electrode and an object to be machined or worked is filled with an electrolyte containing sodium nitrite or sodium chloride and a d.c. current is caused to flow from the object to be worked to the electrode while the electrolyte is caused to flow at a high speed and while electrolytic products are being removed, thus allowing working to be conducted. The electrolytic products include metallic compounds, metallic ions and hydrogen gas dissolved in the electrolyte, and are removed because they act to inhibit a stable electrolytic action (see Japanese Patent Laid-Open Publication No. 61-71921 and Japanese Patent Laid-Open Publication No. 60-44228).
However, such an electrolytic forming method has a serious disadvantage as a means of working. More specifically, it involves the problem that when a three-dimensionally shaped object having a bottom (a three-dimensional recessed structure) is to be worked, even if the gap between the electrode and the object to be worked is kept constant, it is impossible to cause an electrolyte to flow in the gap at a uniform flow rate owing to changes in the resistance of electrolyte flowing in the gap, as determined by the position of the inlet for the electrolyte, the distance of the gap from the inlet, or the extent to which the surface to be worked on the object has a three-dimensional shape which is curved.
In consequence, the extent to which the electrolytic products generated in the gap can be removed differs according to where the removal site is positioned, and the progress of working hence differs also, making it difficult to accurately transfer the electrode to the object to be worked. Therefore, in order to finish the surface to be worked with a high degree of accuracy, another polishing process is necessary, making finishing of the surface to be worked a time-consuming and troublesome task.
Accordingly, in view of the above-described problems of the known systems, the present inventor developed a finishing method using electro-chemical machining, in which in a first stage of the finishing process pulses having a low current density are supplied between the electrode and the object to be worked while they are located in a static electrolyte so as to improve the surface roughness of the surface to be worked and pulses having a high current density are intermittently supplied therebetween so as to remove an oxide layer generated on the surface to be worked, and in which in a second stage of the finishing process pulses having a high current density are supplied so as to obtain a glossy surface (see Japanese Patent Application No. 62-117486).
However, in this finishing method employing electrochemical machining, although pulses having a high current density are supplied for a constant period in the first stage of the finishing process so as to remove the oxide layer, the thickness of the oxide layer generated differs according to the shape of the object to be worked or the type of arrangement employed for removing the electrolytic product and it is therefore difficult to set the oxide layer removing pulse supplying period so as to obtain an optimum working condition. For example, if the supplying period is short and the switch-over to the pulses having a high current density is effected early, the improvement of the surface roughness takes longer, and a longer time is required for processing. On the other hand, if the supplying period is long and if the switch-over to the pulses having a high current density is delayed, an oxide layer having a large electric resistance grows partially on the surface to be worked, changing the density of the current and thereby varying the amount of working conducted per unit area, which leads to inaccuracy in the shape of the worked object as well as in the surface quality.
Further, the timing of the switch-over to the pulses having a high current density must be set in accordance with the shape of the object to be worked, the area to be worked, or the type of arrangement employed for removing the electrolytic products, and working efficiency is thereby reduced.
Accordingly, an object of the present invention is to provide a finishing method employing electro-chemical machining and an electro-chemical finishing machine which enable the timing of a switch-over to pulses having a high current density to be set to provide an optimum working condition and thereby improve working efficiency, and which enable an oxide layer generated on a three-dimensional surface of an object to be worked to be entirely removed so that a highly accurate surface quality (such as a surface having a specular gloss) can be obtained in a short period of time.